The present invention relates to oriented, profiled fibers, the cross-section of which closely replicates the shape of the spinneret orifice used to prepare the fiber. The invention also relates to nonwoven webs comprising the oriented, profiled fibers.
Fibers having modified or non-circular cross-sections have been prepared by conventional fiber manufacturing techniques through the use of specially shaped spinneret orifices. However, correlation between the cross-section of fibers produced from these shaped orifices and the shape of the orifice is typically very low. The extruded polymer tends to invert to a substantially circular cross-section with a gently curved, undulating "amoeba-like" shape rather than the typical crisp, angled shape of the orifice. Numerous workers have proposed specially designed spinneret orifices which are used to approximate certain fiber cross-sections although generally there is little correspondence between the orifice cross-sectional shape and that of the fiber. Orifices are designed primarily to provide fibers with certain overall physical properties or characteristics associated with fibers within general classes of shapes. Orifices generally are not designed to provide highly specific shapes. Specialty orifices have been proposed in U.S. Pat. Nos. 4,707,409; 4,179,259; 3,860,679; 3,478,389; and 2,945,739 and U.K Patent No. 1,292,388.
U.S. Pat. No. 4,707,409 (Phillips) discloses a spinneret for the production of fibers having a "four-wing" cross-section. The fiber formed is either fractured in accordance with a prior art method or left unfractured for use as filter material. The "four-wing" shape of the fiber is obtained by use of a higher melt viscosity polymer and rapid quenching as well as the spinneret orifice design. The orifice is defined by two intersecting slots. Each intersecting slot is defined by three quadrilateral sections connected in series through an angle of less than 180.degree.. The middle quadrilateral sections of each intersecting slot have greater widths than the other two quadrilateral sections of the same intersecting slot. Each slot intersects the other slot at its middle quadrilateral section to form a generally X-shaped opening. Each of the other two quadrilateral sections of each intersecting slot is longer than the middle quadrilateral section and has an enlarged tip formed at its free extremity.
U.S. Pat. No. 4,179,259 (Belitsin et al.) discloses a spinneret orifice designed to produce wool-like fibers from synthetic polymers. The fibers are alleged to be absorbent due to cavities formed as a result of the specialized orifice shapes. The orifice of one of the disclosed spinnerets is a slot with the configuration of a slightly open polygon segment and an L, T, Y or E shaped portion adjoining one of the sides of the polygon. The fibers produced from this spinneret orifice have cross-sections consisting of two elements, namely a closed ring shaped section resulting from the closure of the polygon segment and an L, T, Y, or E shaped section generally approximating the L, T, Y, or E shape of the orifice that provides an open capillary channel(s) which communicates with the outer surface of the fiber. It is the capillary channel(s) that provides the fibers with moisture absorptive properties, which assertedly can approximate those of natural wool. It is asserted that crimp is obtained that approximates that of wool. Allegedly this is due to non-uniform cooling.
U.S. Pat. No. 3,860,679 (Shemdin) discloses a process for extruding filaments having an asymmetrical T-shaped cross-section. The patentee notes that there is a tendency for asymmetrical fibers to knee over during the melt spinning tendency, which is reduced, for T-shaped fibers, using his orifice design. Control of the kneeing phenomena is realized by selecting dimensions of the stem and cross bars such that the viscous resistance ratio of the stem to the cross bar falls within a defined numerical range.
U.S. Pat. No. 3,478,389 (Bradley et al.) discloses a spinneret assembly and orifice designs suitable for melt spinning filaments of generally non-circular cross-section. The spinneret is made of a solid plate having an extrusion face and a melt face. Orifice(s) extend between the faces with a central open counter-bore melt receiving portion and a plurality of elongated slots extending from the central portion. In the counter-bore, a solid spheroid is positioned to divert the melt flow toward the extremities of the elongated slots. This counteracts the tendency of extruded melt to assume a circular shape, regardless of the orifice shape.
U.S. Pat. No. 2,945,739 (Lehmicke) describes a spinneret for the melt extrusion of fibers having non-circular shapes which are difficult to obtain due to the tendency of extruded melts to reduce surface tension and assume a circular shape regardless of the extrusion orifice. The orifices of the spinneret consist of slots ending with abruptly expanded tips. The fibers disclosed in this patent are substantially linear, Y-shaped or T-shaped.
Brit. Pat. 1,292,388 (Champaneria et al.) discloses synthetic hollow filaments (preferably formed of PET) which, in fabrics, provide improved filament bulk, covering power, soil resistance, luster and dye utilization. The cross-section of the filaments along their length is characterized by having at least three voids, which together comprise from 10-35% of the filament volume, extending substantially continuously along the length of the filament. Allegedly, the circumference of the filaments is also substantially free of abrupt changes of curvature, bulges or depressions of sufficient magnitude to provide a pocket for entrapping dirt when the filament is in side-by-side contact with other filaments. The filaments are formed from an orifice with four discrete segments. Melt polymer extruded from the four segments flows together to form the product filament.
It has also been proposed that improved replication of an orifice shape and departure from a substantially circular fiber cross-section can be achieved by utilizing polymers having higher melt viscosities; see, e.g., U.S. Pat. No. 4,364,998 (Wei). Wei discloses yarns based on fibers having cross-sections that are longitudinally splittable when the fibers are passed through a texturizing fluid jet. The fibers were extruded into cross-sectional shapes that had substantially uniform strength such that when they were passed through a texturizing fluid jet they split randomly in the longitudinal direction with each of the split sections having a reasonable chance of also splitting in the transverse direction to form free ends. Better retention of a non-round fiber shape was achieved with higher molecular weight polymers than with lower molecular weight polymers.
Rapid quenching has also been discussed as a method of preserving the cross-section of a melt extruded through a non-circular oriface. U.S. Pat. No. 3,121,040 (Shaw et al.) describes unoriented polyolefin fibers having a variety of non-circular profiles. The fibers were extruded directly into water to preserve the cross-sectional shape imparted to them by the spinneret orifice. This process freezes an amorphous or unoriented structure into the fiber and does not accommodate subsequent high ratio fiber draw-down and orientation. However, it is well known in the fiber industry that fiber properties are significantly improved through orientation. The superior physical properties of the oriented fibers of the present invention enable them to retain their shape under conditions where unoriented fibers would be subject to failure.
The surface tension forces of a polymer melt have also been used to advantage in the spinning of hollow circular fibers. For example, spinnerets designed for hollow fibers include some with multiple orifices configurated so that extruded melt polymer streams coalesce on exiting the spinneret to form a hollow fiber. Also, single orifice configurations with apertured chamber-like designs are used to form annular fibers. The extruded polymer on either side of the aperture coalesces on exiting the spinneret, to form a hollow fiber. Even though these spinneret designs on a casual inspection thus appear to be capable of producing fibers which would significantly depart from a substantially circular cross-section, surface tension forces in the molten polymer cause the extrudate to coalesce into hollow fibers having a cross-section that is substantially circular in shape.
It is also well known in the art that unoriented fibers with non-circular cross-sections will invert from their original shape toward substantially circular cross-sections when subjected to extensive draw-downs at standard processing conditions.
The use of specific polymers as a means of increasing orifice shape retention has also been suggested. Polymers with high viscosity or alternatively high molecular weight [presumably by decreasing flow viscosity] (see Wei above) have been proposed as a means of increasing replication of orifice shape. However, low molecular weight polymers are often desirable at least in terms of processability. For example, low molecular weight polymers exhibit less die swell and have been described as suitable for forming hollow microporous fiber, U.S. Pat. No. 4,405,688 (Lowery et al). Lowery et al described a specific upward spinning technique at high draw downs and low melt temperatures to obtain uniform high strength hollow microfibers.
Significant problems are associated with the techniques that are described for use in forming non-circular profiled shapes particularly with fibers. Highly designed orifice shapes are employed to give shapes that are generally ill defined, merely gross approximations of the actual oriface shape and possibly the actual preferred end shape. The surface tension and flow characteristics of the extruded polymer still tend to a circular form. Therefore, any sharp corners or well defined shapes are generally lost before the cross-sectional profile of the fiber is locked in by quenching.
A further problem arises in that the orientation of the above described fibers is accomplished generally by stretching the fibers after they have been quenched. This is generally limited to rather low draw rates below the break limit. Consequently, where a fiber of a certain denier is desired the die must be at the order of magnitude of the drawn fiber. This significantly increases costs if small or microfibers are sought due to the difficulties in milling or otherwise forming extremely small orifices with defined shapes. Finally, using a rapid quench to preserve shape creates an extremely unoriented fiber (see Shaw et al.) sacrificing the advantages of an oriented fiber for shape retention.
A general object of the present invention seeks to reconcile the often conflicting objectives, and resulting problems, of obtaining both oriented and highly structured or profiled fibers.